Bringing a chlorine-containing tin compound in a gas or spray form into contact with a heated glass body to form a tin oxide layer on the surface of the glass body has been widely carried out (see, for example, Japanese Patent Laid-Open No. 19895/1984 or Japanese Patent Laid-Open No. 131547/1991). In these cases, however, NaCl produced by a reaction of sodium (Na) constituting the glass with chlorine (Cl) derived from the coating agent, together with tin oxide layer, is often deposited on the surface of glass. Thereafter, NaCl is dropped from the surface to create pinholes which constitute defects of the layer, resulting in lowered fastness properties of the layer and the creation of haze.
On the other hand, a method for coating an organotin compound, a chlorine-free tin agent, onto glass has also been reported (see Japanese Patent Publication No. 11234/1970). Use of the chlorine-free coating agent leads to an expectation that the creation of pinholes on the coating can be inhibited. So far as the present inventors know, however, in the case of the conventional coating method using the organotin compound, it is difficult to apply the method to continuous production of coated glass and, in addition, to put the method to practical use. In fact, Japanese Patent Publication No. 11234/1970 is silent on continuous production associated with coating.
According to studies conducted by the present inventors, the difficulty of continuously producing coated glass in the method described in Japanese Patent Publication No. 11234/1970 is considered to derive from the following facts.
Firstly most of the organotin compounds disclosed in Japanese Patent Publication No. 11234/1970 have unsatisfactory vaporization properties. Since the organotin compounds are liquid or solid at room temperature, they are generally heat vaporized and carried onto the surface of glass. In this case, poor vaporization properties lead to a chemical change of the organotin compound before vaporization. As a result, a part of the reactant is left within a vaporizer. This creates disadvantages, such as lowered vapor deposition efficiency due to a reduction in the amount of the material deposited on the glass based on the amount of the starting material introduced and the difficulty of repeatedly using the vaporizer in a continuous manner due to the residue within the vaporizer. For this reason, the practicability is lowered.
Further, in the surface having a relatively low temperature (temperature about 150 to 300° C.) called a “cold wall” within piping for carrying the starting material, the vaporized organotin compound in its ester bond sites and the like is influenced by water contained in the air and consequently causes a chemical reaction which often produces by-products due to the decomposition or polymerization of the organotin compound. The by-products are in many cases deposited or accumulated on the inner wall of the piping. Therefore, when the amount of the deposited or accumulated by-products exceeds a certain level, the operation of the production line should be stopped to remove the deposited or accumulated by-products. This makes it very difficult to carry out continuous production using the organotin compound.
Further, the organotin compound described in Japanese Patent Publication No. 11234/1970 cannot be used in a production line for glass coating in a system open to the air without difficulty. For example, when dibutyltin diacetate is used as the coating agent, a tin oxide layer having high quality and excellent fastness properties and free from haze can be applied onto the surface of glass under specific conditions, for example, in a closed system with the humidity being controlled at a certain value or less. On the other hand, it was confirmed that, in coating operation in a system open to the air, water in the air causes the decomposition or polymerization of dibutyltin diacetate in the vaporizer or on the inner wall of piping for the coating agent and on other cold walls and the resultant decomposition or polymerization product is deposited or accumulated thereon. The production of the above reaction product indicates that it is difficult to recover dibutyltin diacetate remaining unreacted and not deposited on the surface of glass. This makes it difficult to recycle the coating agent containing dibutyltin diacetate in a water-containing system open to the air.
In the actual glass production line, in many cases, coating is continuously carried out while carrying heated glass. The step of coating glass in the production line cannot be carried out without difficulty in a closed system rather than in a system open to the air and thus is unrealistic also from the viewpoint of economy.
In the system open to the air, a variation in components (particularly water) of the air, temperature, and pressure in the air depending upon seasons is indispensable. Therefore, the production of by-products by the above reaction is unavoidable.
For this reason, a glass coating method has been desired which is compatible with the conventional glass production process, can be applied to a system open to the air, and can continuously produce glass coating. Further, a glass coating agent has also been desired which can be utilized in the above glass coating method and can form a high-quality layer having excellent fastness properties and free from haze.